Add support for tag templates
Tags specified in PUBLISH_IMAGE_TAG and PUBLISH_IMAGE_EXTRA_TAGS (and their command-line equivalents) now undergo mild template processing before being used. At this time there is only one template replacement:
%DATE% The UTC build date in %Y-%m-%d-%H%M%S format.
For example if PUBLISH_IMAGE_TAG is '%DATE%-${BUILD_VARIANT}', the resulting tag will be something like 2023-06-06-094537-production (if BUILD_VARIANT is "production").
Bug: T338224